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Old 24-07-2013, 13:40   #1
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12V Relay Amp Ratings?

Looking at some typical twelve volt power relays, similar to the common Bosch "30A" 12v spdt type, I see that a lot of them are no longer rated with just one amp rating. Instead, they typically are rated "50A/30A" or "40A/20A" and one maker (NTE) explains that as "50A for the NO contacts, 30A for the NC contacts". Hella rates the same way, these are not nameless brands.

So in other words, the relay contacts are asymmetric now, and the contact for the NC position is not rated anywhere near what the NO position is rated for. Which just seems plain foolish to me, especially since so many of them are advertised JUST by the larger NO rating, which is not the rating they'd normally be used in service for.

Did I miss the memo? Have standards always called for this or are the makers just cheaping out and playing ratings inflation now?

Waiter, my reality check please?
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Old 24-07-2013, 13:46   #2
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I believe the higher rating is for making contact and the lower rating is for braking contact. At least, that is how we designed circuits around them; I had little patience with these half mechanical devices with springs and stuff.

EDIT: you get arcing when opening contacts much sooner then when closing contacts.
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Old 24-07-2013, 14:05   #3
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Re: 12V relay amp ratings?

Nick, NTE was very specific about this. Theirs are not make/break ratings, but separate ratings for the NO/vs/NC contacts. Which seems ludicrous to me but in this day and age, when a gallon contains 56 ounces and a pound of coffee only 10.5-12 ounces...I'm used to that.

Make/break ratings, AC vs DC, arcing, all these things are nice but apparently this is something else they are doing.
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Old 24-07-2013, 16:34   #4
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Re: 12V relay amp ratings?

Just guessing here... But since a SPDT relay is often used as a SPST relay it might make sense to have the NO contact be designed to carry the larger current. It seem less likely that someone would want to supply high current continuously through a relay in a 12V system. That level of current seem more suited to a solenoid.
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Old 24-07-2013, 17:00   #5
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Re: 12V relay amp ratings?

That would make sense, but then why not save a penny more and just sell it as an SPST relay? Why skimp on just the one contact?? Heck, cut out the contact and the qd fitting and you can even save two cents on each relay. Maybe three.

How many accountants do you have to keelhaul before they rub all the barnacles off a hull anyhow??
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Old 24-07-2013, 17:47   #6
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Re: 12V relay amp ratings?

Maybe it is just cheaper to make, package, stock, and distribute a standard product at a higher cost to the user then to do the same for two standard products. The rating difference could be the result of upgrading an old product to meet new requirements at the lowest possible cost. Again, this guessing.
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Old 25-07-2013, 07:43   #7
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Re: 12V Relay Amp Ratings?

It is nothing new. Most SPDT relays have a spring return that holds the NC contact closed and a solenoid to pull the armature and contact to close the NO side. There has to be enough magnetic force to overcome the spring when it first turns on but once the armature seats closed there is 10 times as much force than is needed so greater pressure on the NormallyOpen contact than is provided by just the spring on the NormallyClosed giving a higher current capacity for the NO contact.

In fact you can reduce the current in the coil by as much as 80% once the contact is closed to reduce power consumption and heating. Some of our larger Combiners have this feature although the smaller ones only use about 1 to 2 watts so there is little to gain for the extra controls.
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Old 25-07-2013, 16:47   #8
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NC contacts are almost invariably rated lower then NO. this is because destructive arcing occurs when the contact starts opening not when it starts closing. ( ie inductive effects. )
Dave
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Old 25-07-2013, 18:02   #9
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Re: 12V Relay Amp Ratings?

Thanks, Andina, and Dave.

That makes sense. If I'm reading between the lines correctly, the actual contacts are the same then, but one is derated because it will see more severe conditions even though it is physically the same part. So they're not cheating on the parts, but the ratings are recognizing that one will be more severely stressed and should be treated more kindly.

Yes?

As it turns out I am using the relay as SPST anyway, so the lower rating for the unused contact won't matter in this application. Nice to know and understand for the future though.

Ah, for the old days an mercury-wetted relays.<G>
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Old 25-07-2013, 21:52   #10
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Re: 12V Relay Amp Ratings?

Quote:
Originally Posted by goboatingnow View Post
NC contacts are almost invariably rated lower then NO. this is because destructive arcing occurs when the contact starts opening not when it starts closing. ( ie inductive effects. )
Dave
No, both the NO and the NC contacts both close and open their respective circuits. The amount of arcing is a function of the circuit parameters, not which contact is being used.

The lower rating of the NC contact is due to the lower mechanical contact pressure from the spring return compared to the magnetic force on the NO contact.
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Old 26-07-2013, 03:02   #11
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Quote:
Originally Posted by Andina Marie View Post
No, both the NO and the NC contacts both close and open their respective circuits. The amount of arcing is a function of the circuit parameters, not which contact is being used.

The lower rating of the NC contact is due to the lower mechanical contact pressure from the spring return compared to the magnetic force on the NO contact.
That must be it. The other option of arcing on opening was the first thing I had mentioned already but manufacturers said the contact itself. With contacts being the same normally, it must indeed be the force in each direction that differs.... but I haven't worked out which way yet...

EDIT: the NO opens with the spring while the NC opens with the magnet. That would make the NC quicker and stronger in opening, not the NO. But.. the NO using the magnet, has the spring working against it, which must be overcome with the magnet.

I'm pretty sure we must look at opening of contacts for maximum ratings, not the closing of contacts.
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Old 26-07-2013, 03:31   #12
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NO uses the magnet to close and therefore has higher contact pressure thus higher current rating.
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Old 26-07-2013, 06:51   #13
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NO uses the magnet to close and therefore has higher contact pressure thus higher current rating.
I don't agree. When NO or NC cntacts are closed, they can transfer many times the current that they are rated for. It is when they must make or break that circuit that is critical. To open cntacts when max. Current is flowing is the big problem for the contact; this is the moment they burn.
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Old 26-07-2013, 09:37   #14
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Re: 12V Relay Amp Ratings?

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Originally Posted by s/v Jedi View Post
I don't agree. When NO or NC cntacts are closed, they can transfer many times the current that they are rated for. It is when they must make or break that circuit that is critical. To open cntacts when max. Current is flowing is the big problem for the contact; this is the moment they burn.
Jedi, both the NO contact and the NC contact have to make and break current.
Perhaps you misunderstand the NO and NC meaning.
The NC contact is closed when there is no power on the relay coil, it connects the armature to the NC terminal but although the relay is idle this contact is closed and can carry current.

The NO contact is closed when power is applied to the coil. That breaks the NC connection and closes the NO contact.

Both have closing functions which can involve heavy current.
BOTH HAVE opening functions that can cause arcing.
If the current is the same, both have identical problems with inrush current and arcing.
In fact if you get down to the nitti gritti when a NO contact closes it does so with higher power than the return spring of a NC contact so the extra speed and acceleration at the last moment causes much more contact bounce that you see at the gentler closing of a NC contact closing. This means breaking the inrush current and arcing a number of times on the NO contact.

As I've indicated previously, in over 20 years we have never had a Combiner relay contact failure.

One fishing boat who had been using our old 150 amp Combiner complained that his Combiner had always BUZZED when the batteries were around 13.2 volts and that was annoying. It had been running for 5 years with the timing capacitor not functioning so instead of limiting operation to 1 per minute it was doing 10 per second. So 5 years running was the equivalent of 3,000 years normal use. He sent it back and the relay contacts were about 50% eroded. I put in a new timing capacitor and returned it to him, should be good for at least another 2,000 years .
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Old 26-07-2013, 13:29   #15
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Quote:
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Jedi, both the NO contact and the NC contact have to make and break current.
Perhaps you misunderstand the NO and NC meaning.
The NC contact is closed when there is no power on the relay coil, it connects the armature to the NC terminal but although the relay is idle this contact is closed and can carry current.

The NO contact is closed when power is applied to the coil. That breaks the NC connection and closes the NO contact.

Both have closing functions which can involve heavy current.
BOTH HAVE opening functions that can cause arcing.
If the current is the same, both have identical problems with inrush current and arcing.
In fact if you get down to the nitti gritti when a NO contact closes it does so with higher power than the return spring of a NC contact so the extra speed and acceleration at the last moment causes much more contact bounce that you see at the gentler closing of a NC contact closing. This means breaking the inrush current and arcing a number of times on the NO contact.

As I've indicated previously, in over 20 years we have never had a Combiner relay contact failure.

One fishing boat who had been using our old 150 amp Combiner complained that his Combiner had always BUZZED when the batteries were around 13.2 volts and that was annoying. It had been running for 5 years with the timing capacitor not functioning so instead of limiting operation to 1 per minute it was doing 10 per second. So 5 years running was the equivalent of 3,000 years normal use. He sent it back and the relay contacts were about 50% eroded. I put in a new timing capacitor and returned it to him, should be good for at least another 2,000 years .
Andina, no need to explain me NO vs NC, I am a retired electronics designer. But I could need an explanation why NO gets a higher rating. Like I wrote a number of times already, I think that opening of the contact (any contact) while it is carrying max. rated current (i.e. arcing), is the limiting factor for it's rating, followed by closing of contacts. This is why I only look at what happens when a contact opens. Pls. Explain why when you do not agree with that.
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